All modern aircraft employ some type of landing gear assembly for supporting the aircraft fuselage above the ground when the aircraft is not in flight. Virtually all landing gear assemblies include some type of shock absorption system. These shock absorption systems are not only necessary for normal landings, but are also critical for hard and/or crash landings.
The landing gear in a rotorcraft, such as a helicopter, must be able to protect the airframe and mission equipment during hard landings. Further, the landing gear must be able to increase the survivability of the rotorcraft occupants during crash impacts. This is accomplished by designing the landing gear to absorb a significant part of the impact energy during the hard or crash landing.
Rotorcraft landing gears are calibrated in terms of sink speed. Military rotorcraft are required to function in vertical sink speed ranges from 10 ft/sec for normal landings to 42 ft/sec for crash landings. In order to provide protection to the airframe, mission equipment package, and occupants over this wide range of impact sink speeds, landing gear having multiple-stage shock struts have been utilized.
A common multiple-stage shock strut employs a first-stage nitrogen-oil type oleo. The first stage functions in landing sink speeds up to the reserve energy condition of about 12 ft/sec. During such low landing sink speeds, the second-stage is static. However, during landing sink speeds that exceed the reserve energy condition, including crash conditions, the second-stage of the shock strut is activated.
One way to activate the second-stage involves the controlled mechanical failure of design features within the shock strut such as an internal diaphragm, shear pin, or shear collar. Prior to the controlled mechanical failure, the second-stage is static and does not stroke. However, upon the controlled mechanical failure, the second-stage of the shock strut strokes and absorbs the landing force. While most shock strut second-stages include a stroking piston similar to the first stage, some landing gear shock strut second-stage designs consist entirely of mechanical devices such as crushable tubes.
The major limitation of current military rotorcraft landing gear is that each shock strut assembly is designed for a specific aircraft gross weight. The landing gear performance degrades when it is employed in an aircraft having a higher gross weight. This may result in increased airframe and mission equipment package damage during hard and/or crash landings. Therefore, derivative or second generation rotorcraft having increased gross weights require the redesign and requalification testing of suitable landing gear. Unfortunately, redesigning and requalifying landing gear is expensive and time-consuming.
In view of the foregoing, it would be desirable to provide a landing gear assembly including a variable force activation device for enabling it to be employed on aircraft having different gross weights. As such, the same landing gear could be used for a family of aircraft designs without requiring landing gear redesign or requalification.